The invention applies to a device and the use of such a device for the determination of the density of a plasma.
Plasmas—electrically activated gases—find use in a variety of technical fields; their particular physical properties are frequently the basis of innovative products and processes. The exact supervision and—in the case of deviations—the adjustment of the plasma state are essential for the success of processes which are based on the use of technical plasmas. An important parameter of plasmas is the space and time dependent electron density ne. To know its value is essential for the characterization of plasmas. However, in technological plasmas, particularly in reactive plasmas, the determination of the electron density is difficult.
The determination of the plasma density (and of other plasma parameters) is subject of a scientific discipline of its own, plasma diagnostics. A number of diagnostic methods have already been developed and employed. Examples are optical methods, which come in a wide variety. A first classification distinguishes emission spectroscopy, absorption spectroscopy, and fluorescence spectroscopy. Mass spectroscopy and plasma monitoring are particle diagnostic methods. The recording of V/I characteristics, the use of Langmuir probes, and microwave interferometry belong to electrical diagnostics
Of these methods, however, only a few are industry-compatible. The notion of “industry compatibility” refers to a number of important requirements for the applicability of a diagnostic method in production lines and other industrial environments: Robustness of the method against contamination and perturbations, no interference with the monitored process, low complexity of the diagnostic process and its evaluation, online capability. Low cost with respect to investment and maintenance is also important. Process end-point detection and the identification of hardware faults are particular industrial measurement tasks.
A promising method for industrial plasma diagnostics is plasma resonance spectroscopy. In this method, a high-frequency signal in the Giga-Hertz range is coupled into the plasma. The signal reflection is measured as a function of the frequency. In particular the resonances—maxima of the absorption—are determined. The location of these maxima is a function of the desired central plasma parameter, the electron density. At least in principle, it can be determined this way in an absolute and calibration-free manner. High-frequency measurements have little or no influence on the technical process, and are to a large extent insensitive against contamination. Their requirements on investment and maintenance are very small. Plasma resonance spectroscopy is characterized by simple system integration properties, high measurement speed, and good online capabilities. A disadvantage of plasma resonance spectroscopy is that a mathematical model is required to evaluate of measurement (i.e., to calculate the electron density from the resonance curve). In addition, particular technology is required for the spatial resolution of the measurement (i.e., for the determination of the electron density as a function of the position).
In various publications (U.S. Pat. No. 6,339,297 B1, U.S. Pat. No. 6,744,211 B2), Sugai et al. disclosed a method for measuring the plasma density on the basis of resonance spectroscopy, and described a particular design of an absorption probe. The probe consists of a dielectric tube, closed at one end, open at the other. The closed end of the probe is located in the plasma, while the open end is located outside of the plasma chamber. A coaxial cable acting as an antenna is inserted into the tube.
The plasma absorption probe proposed by Sugai et al. has a convincingly simple design. The evaluation of the signal, however, is problematic: It is difficult to deduce the really interesting quantity, the electron density of the plasma, from the measured primary signal (the frequency curve of the absorption).
The underlying reason can be understood from a theoretical analysis of the absorption diagnostic method. The probe is represented by a system of two electrodes A and B, which are introduced into a spatially bounded region (see FIG. 10). The boundary is typically formed by a grounded wall, i.e., by a surface W which has the high-frequency potential zero. The bounded region contains dielectric and plasma with an at least partially unknown distribution. (More exactly: The unknowns are the distribution of the plasma, and the thickness of the plasma boundary layer which is produced by the plasma itself and which acts as dielectric.) When high-frequency voltages are applied to the two electrodes, currents can be determined and analyzed as function of the frequency. On the basis of this abstract model one ca demonstrate theoretically that the response of the probe, which is relevant for the measurement of the electron density, can be described as the superposition of isolated resonances (modes). This is illustrated by the schematic electrical circuit diagram in FIG. 11, which represents each of the modes by an LCR series resonance circuit. Obviously, there exists coupling between the two electrodes (A to B), as well as coupling between the respective electrodes and the wall (A to W, and B to W, respectively.)
The schematic electrical circuit diagram demonstrates the disadvantages of the previous method according to Sugai et al.:                The resonance characteristics results from the superposition of an infinite number of sub-modes. Practically, it is not possible to determine the corresponding resonance circuit parameters from the primary measurement curve (which has only limited accuracy).        Even if the parameters were determinable, it would be impossible in practice to determine the actual plasma density: Although the parameters could be calculated for a given density with considerable effort, but this would not solve the “inverse problem” in a measurement.        In the resonance characteristics, the coupling between the electrodes is superimposed on the coupling to the distant wall. The latter correspond to a collective excitation of the entire plasma and hence do not only involve the electron density at the probe location. A spatial resolution of the measurement thus becomes impossible.        
EP 0 692 926 A1 discloses a diagnostic method which analyses the current-voltage characteristics of a probe introduced in a low pressure plasma. This is essentially a variant of a Langmuir probe, with a modification that prevents perturbations of the current-voltage characteristics caused by high-frequency with a suitable device.
EP 0 719 077 A1 describes a diagnostic method which is known under the name SEERS (self-excited electron resonance spectroscopy). In this method, the electron density in a low-pressure plasma is measured by using a resonance. The method is passive. It utilizes the self-excitation of a resonance in an HF plasma which results from a nonlinear interaction of the high-frequency power, which supplies the energy, with the plasma boundary layer. The method is therefore only suitable for asymmetric HF discharges. Collective, rather than local, excitation modes are observed. Thus, the method does not allow for spatial resolution. Consequently, not a probe, but a wall sensor is used.
DE 696 05 643 T2 describes a device for measuring the ion flux onto a surface exposed to a low-pressure plasma. This method does not use spectral techniques. The resonance phenomenon is also not utilized. Instead, the method is based on measuring the discharge rate of a capacitor which is placed between an HF voltage source and a probe in form of a plate in contact with the plasma.
DE 42 00 636 A1 describes the high-frequency compensation of an electrical Langmuir probe. It is proposed to utilize the probe cable as part of the circuit which suppresses the high-frequency. This allows placing the other elements of the circuit farther away from the probe tip, outside of the reactor. No frequency-tunable high-frequency is introduced, and no spectral measurements are performed. Instead, the method evaluates a DC current-voltage characteristics. The invention is directed to a method for compensating the perturbation of this curve by superimposed high-frequency.
DE 40 26 229 C2 proposes to prevent coating of an electrical Langmuir probe in reactive plasmas by heating. The probe is here alternatingly connected by a cyclically operated switch with a measurement circuit and a heater power supply. Also with this method, no frequency-tunable high-frequency is supplied and no spectral measurements are performed. Instead, the method evaluates a DC current-voltage curve. The technical core concept is to provide a method for preventing the perturbation of the curve by layers deposited by the plasma. To describe the state of the art, the following publication should also be mentioned: J.-C. Schauer, S. Hong, J. Winter: “Electrical measurements in dusty plasmas as a detection method for the early phase of particle formation”, Plasma Sources Sci. Technol. 13 (2004) 636-645.